linux-kernel - Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO scheduler

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Date:   Fri, 20 Jan 2017 07:28:26 -0700
From:   Jens Axboe <axboe@...com>
To:     Paolo Valente <paolo.valente@...aro.org>
CC:     <linux-block@...r.kernel.org>,
        Linux-Kernal <linux-kernel@...r.kernel.org>, <osandov@...com>
Subject: Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline
 IO scheduler

On Fri, Jan 20 2017, Paolo Valente wrote:
> 
> > Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@...com> ha scritto:
> > 
> > This is basically identical to deadline-iosched, except it registers
> > as a MQ capable scheduler. This is still a single queue design.
> > 
> 
> Jens,
> no spin_lock_irq* in the code.  So, also request dispatches are
> guaranteed to never be executed in IRQ context?  I'm asking this
> question to understand whether I'm missing something that, even in
> BFQ, would somehow allow me to not disable irqs in critical sections,
> even if there is the slice_idle-expiration handler.  Be patient with
> my ignorance.

Yes, dispatches will never happen from IRQ context. blk-mq was designed
so we didn't have to use irq disabling locks.

That said, certain parts of the API can be called from IRQ context.
put_request and the completion parts, for instance. But blk-mq doesn't
need to grab any locks there, and neither does mq-deadline. This might
be different from bfq. lockdep can be a big help there.

-- 
Jens Axboe